Raster Image Processors (RIPs) are utilized to determine which pixels will be used to render a given printed image. Multi-pass inkjet printing re-creates such an image on the medium as it is incremented progressively under the print heads after each successive print swath until all pixels are addressed to complete the print. Interleaf print masking is used to determine which printhead nozzles will be used to address each pixel location for a given print mode. Each raster line is typically printed such that successive pixel locations in the carriage scan axis may be addressed by a number of nozzles spaced vertically apart, thus softening the effect of any nozzle jetting anomalies. The number of nozzles used in each rasterline is typically proportional to the number of print passes set by the print mode. The number of swaths required to print a completed image is generally equal to the number of passes in the print mode.
The use of one phase-locked leaf mask tends to result in different color ink drops landing on each other at each pixel location addressed in a given printed image. Further, during bi-directional printing, the order that different color dots are laid down would alternate on each print swath for printheads mounted side by side. For instance, in 3-pass printing the first and third swath would consist of magenta over yellow dots, while the second swath would consist of yellow over magenta dots. This tends to produce obvious hue shift banding in three-pass mode at the media feed increment because images are printed using two right to left swaths for each left to right swath.
Apart from the color differences that result from the ink drop color lay-down order, there is a strong tendency for dots laid down in contact with each other to coalesce or flow together, especially during high speed printing. This coalescence can change the apparent density, color and texture of image areas and generally is not a desirable ink interaction. For a given ink and media combination and print mode, coalescence depends on factors that include image saturation, ambient temperature and humidity, color calibration, and ink drop jetting directionality and velocity variations. This means that coalescence tends to be uncontrolled and can be exaggerated by small factors.
In practice images are stochastically screened to improve smoothness by introducing some dot placement randomization. This reduces the number of different color dots deliberately fired to the same pixel location in image areas with lower color saturation, but this becomes less effective as total saturation rises where a higher proportion of primary color dots share pixel locations. When conventional leaf masking is used, this tends to exaggerate coalescence and hue shift banding in areas of high saturation.
The present invention meets these needs by improving print quality at high speed, saving time and energy.